The present invention relates to a magnet, for example a magnetic assembly suited for magnetic resonance imaging apparatus, and more particularly to a homogeneous magnetic field generating apparatus having an open type magnet with a particularly wide opening.
A magnetic resonance imaging apparatus (hereinafter referred to as an MRI apparatus) produces an image showing physical and chemical properties of a subject under inspection by utilizing a nuclear magnetic resonance phenomenon that occurs when the subject placed in a homogeneous static magnetic field is irradiated with an electromagnetic wave. The MRI apparatus is used particularly for medical treatments.
The MRI apparatus mainly includes a static magnetic field generating unit to apply a homogeneous static magnetic field in an imaging volume, an RF coil system to transmit and receive an electromagnetic wave, and a gradient magnetic field generating unit to apply a gradient magnetic field that gives position information on a resonant phenomenon.
The MRI apparatus can be divided into two general categories mainly in terms of a magnetic field application method. One is a horizontal magnetic field apparatus in which an imaging volume is placed in an internal space of a group of coils arranged coaxially in multiple layers, and the other is a vertical magnetic field apparatus (open type) in which an imaging volume is sandwiched between opposing coil groups. Because of its openness the vertical magnetic field apparatus reduces a psychological burden on a patient and significantly improves an accessibility to the patient by an inspector.
In the MRI apparatus, an increase in the intensity of static magnetic field results in an increased resonant signal strength, which in turn leads to a reduced imaging time and a higher function imaging. Therefore, efforts are being made to enhance the intensity of magnetic field. Simply increasing a magnetomotive force of a magnetic field source for a higher magnetic field strength naturally increases leakage fluxes. The open type MRI apparatus employs a method of confining or canceling the magnetic flux leaking from a source of magnetic field to reduce the leakage fluxes.
As the method for reducing leakage fluxes there are mainly three types: a passive shield type, an active shield type and a toroidal coil type (JP-A-10-4010). The toroidal coil type, for example, is described in JP-A-10-4010.